Method of making a high impedance surface

ABSTRACT

A high impedance surface and a method of making same. The surface includes a molded structure having a repeating pattern of holes therein and a repeating pattern of sidewall surfaces, the holes penetrating the structure between first and second major surfaces thereof and the sidewall surfaces joining the first major surface. A metal layer is put on said molded structure, the metal layer being in the holes, covering at least a portion of the second major surface, covering the sidewalls and portions of the first major surface to interconnect the sidewalls with other sidewalls via the metal layer on the second major surface and in the holes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional application of U.S. Ser. No.09/905,794, filed on Jul. 13, 2001 now U.S. Pat. No. 6,739,028.

TECHNICAL FIELD

This invention improves upon current techniques for manufacturing highimpedance surfaces which surfaces are also known as resonant texturedground planes or a “Hi-Z” surfaces and which surfaces are presently madeusing printed circuit board techniques. The present invention providesnew methods of manufacturing such surfaces based on molding and/orrelated techniques, and also provides several structures that aremanufacturable using these techniques. The invention allows Hi-Zsurfaces to be mass-produced more rapidly and at a lower cost than theprior art techniques, which primarily involve printed circuit boardtechnology. This invention also provides a Hi-Z structure in which thecapacitors are vertical, instead of horizontal, so that they may betrimmed after manufacturing, for tuning purposes.

BACKGROUND OF THE INVENTION

Recently, a new kind of electromagnetic ground plane has been developedwhich is known as a high-impedance or Hi-Z surface. See D. Sievenpiperand E. Yablonovitch, “Circuit and Method for Eliminating SurfaceCurrents on Metals” U.S. provisional patent application, Ser. No.60/079,953, filed on Mar. 30, 1998 by UCLA and a related PCT applicationpublished as WO 99/50929 on Oct. 7, 1999. This prior art structureconsists of a metal ground plane covered with an array of tiny resonantcavities. These resonant cavities alter the effective electromagneticimpedance of the surface, so that it appears to have a high impedance(>>377 ohms), instead of a low impedance (≈0 ohm) like an ordinary metalsurface. Because of its high impedance, the Hi-Z structure can support afinite tangential electric field at its surface, which is not possiblewith a smooth metal ground plane. This textured surface is important forvarious applications in the field of antennas. In particular, it isuseful for low-profile antennas because radiating elements can be placeddirectly adjacent to the Hi-Z surface (i.e. spaced less than <<0.01wavelength therefrom) without being shorted out. This provides anadvantage compared to an ordinary metal ground plane, which normallyrequires a separation of roughly ¼ wavelength between the ground planeand the antenna, resulting in antennas that are at least ¼ wavelengththick. In addition to providing a way to produce very thin antennas, theHi-Z surface also suppresses surface currents, which tend to interferewith the performance of the antenna by propagating across the groundplane and radiating from edges, comers, or other discontinuities. Theradiation produced by these surface currents combines with the directradiation from the antenna, and produces ripples in the radiationpattern, as well as significant radiation into the backward directionbehind the ground plane. By suppressing these surface currents, one canproduce antennas with much smoother radiation patterns, and with lessbackward radiation. In short, the antennas are both more compact andmore efficient when made with a Hi-Z surface.

The Hi-Z structure can be most easily understood by considering theeffective circuit that describes the resonant cavities. In the structureshown in FIG. 1, the Hi-Z surface is constructed as a lattice ofoverlapping “thumbtack”-like protrusions on a flat metal ground plane22. The protrusion consist of flat metal plates 10 connected to theground plane by metal plated vias 13. This prior art structure shownhere is built using printed circuit board techniques. The printedcircuit board is not shown for ease of illustration, but the flat metalplates 10 would appear on the printed circuit board's top surface whilethe ground plan 22 is disposed on its bottom surface. The capacitance ofthe structure is determined by the proximity and overlap area of themetal plates 10. The inductance is controlled by the area of the currentloop that connects adjacent plates, which is primarily determined by thethickness of the structure. The resonance frequency of the surface isthen given by

$\omega = {\frac{1}{\sqrt{LC}}.}$Near the resonance frequency, the surface has high impedance, and cansuppress the propagation of surface currents. The bandwidth of thesurface, or the frequency band where the impedance is greater than 377ohms, is given by

${BW} = {\frac{\sqrt{L/C}}{\sqrt{\mu_{o}/ɛ_{o}}}.}$This roughly determines the bandwidth of antennas that can be built onthese surfaces.

Typically, in the prior art, Hi-Z surfaces are produced by printedcircuit board techniques. In order to achieve a low resonant frequency(<10 GHz or so) in a thin structure (a few mm thick), a large amount ofbuilt-in capacitance is required. This is accomplished using amulti-layer structure, in which the capacitors are of a parallel-plategeometry. The vias 12 are made by drilling through both boards, and thenplating the inside of the holes with metal 13. The steps taken infabrication are shown in FIGS. 2( a)–2(f). First, two printed circuitboards, one relatively thick and one relatively thin form the startingmaterials (see FIG. 2( a)). The inner layers are patterned (see FIG. 2(b)), and the boards are bonded together (see FIG. 2( c)). Then holes 12are drilled through the structure to define the positions of the vias(see FIG. 2( d)). These are then plated with metal 13 (see FIG. 2( e)).Finally, the outer layers are patterned (see FIG. 2( f)). The mosttime-consuming and expensive task is drilling the vias 12. A fastcomputer-controlled drill can drill on the order of one hole per second.Typical lattice periods for these structures are on the order of ¼ inch,which means that the total drilling time can approach one hour persquare foot.

What is needed is a method of producing a similar structure by fasterand more economic techniques, in which the holes do not need to bedrilled individually, but instead can be produced en masse by some othertechnique. This invention provides techniques for producing such astructure by molding, as well as new geometries that are amenable tosuch manufacturing techniques. The resulting structure is less expensiveand less time-consuming to fabricate. Furthermore, it has the additionalbenefit that certain embodiments thereof can be tuned after fabricationto adjust for variations in the manufacturing process. This feature alsoallows a single mold to be used to build structures with slightlydifferent resonant frequencies.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a Hi-Z surface that can be produced byinjection molding, which permits large areas to be produced rapidly andat a low cost. Additionally, certain embodiments of the structure arealso technically superior in that they can be tuned after manufacturing,to adjust for variations in the manufacturing process, thus allowing asingle mold to be used for structures with slightly different resonancefrequencies, and/or allowing different areas of a single Hi-Z surface tobe tuned to different resonance frequencies.

In one aspect the present invention provides a method of making a highimpedance surface comprising the steps of: molding a structure from adielectric material to form the structure, the structure having aplurality of holes therein and a plurality of ridges on at least onemajor surface of the structure, the ridges having sidewalls; plating thestructure, including the interiors of the holes therein and thesidewalls, with a layer of metal; removing at least a portion of thelayer of metal which bridges across the ridges to thereby definecapacitor plates on the sidewalls.

In another aspect the present invention provides a method of making ahigh impedance surface comprising the steps of: molding a structure froma dielectric material to form the structure, the structure having aplurality of holes therein and a plurality of trenches on at least onemajor surface of the structure, the trenches having sidewalls and bottomwalls; and plating the structure, including the interiors of the holestherein and the sidewalls, but not the bottom walls of the trenches,with a layer of metal.

In still yet another aspect the present invention provides a method ofmaking a high impedance surface comprising the steps of molding astructure from a dielectric material, the structure having a first majorsurface, a second major surface, a plurality of holes which penetrateboth major surfaces, and a plurality of sidewall features on the firstmajor surface; and applying at least one metal layer to the structure inthe interiors of the holes therein, on the sidewall features, and on thesecond major surface, the at least one metal layers on the sidewallfeatures defining plates of capacitors which are connected toneighboring plates of capacitors via the at least one metal plate in theholes and on the second major surface.

In still yet another aspect the present invention provides a method ofmaking a high impedance surface comprising the steps of molding astructure from sheet metal, the structure having a plurality of openingstherein with confronting sidewalls on the sides of the openings, thestructure also having a plurality of protrusions projecting from a majorsurface thereof; and joining the structure to additional sheet metalsuch that ends of the protrusions remote from the major surface arecoupled to the additional sheet metal.

In yet another aspect the present invention provides a high impedancesurface comprising a molded structure having a repeating pattern ofholes therein and a repeating pattern of sidewall surfaces, the holespenetrating the structure between first and second major surfacesthereof and the sidewall surfaces joining the first major surface; and ametal layer on the molded structure, the metal layer being disposed inor filling the holes, covering at least a portion of the second majorsurface, covering the sidewalls and portions of the first major surfaceto interconnect the sidewalls with other sidewalls via the metal layeron the second major surface and in the holes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a prior art Hi-Z surface;

FIGS. 2( a)–2(f) depict the manufacturing steps used in making a priorart Hi-Z surface;

FIG. 3( a) is a side sectional view through a structure which acts as aform for making a Hi-Z surface in accordance with the present invention,the section line therefor being shown in FIG. 3(b);

FIG. 3( b) is a plan view of the structure shown in FIG. 3( a);

FIGS. 4( a)–4(c) show the structure of FIGS. 3( a) and 3(b) beingcovered by a metal and then the metal being partially removed to definethe capacitor plates;

FIG. 4( d) shows the embodiment of FIG. 4( b) with an addedplanarization layer;

FIG. 4( e) depicts an alternative embodiment wherein the opposingcapacitor plates formed on the sidewalls are non-parallel;

FIGS. 5( a)–5(f) depict another embodiment of a Hi-Z surface;

FIGS. 6( a)–6(e) depict still another embodiment of a Hi-Z surface; and

FIGS. 7( a)–7(d) depict yet another embodiment of a Hi-Z surface.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT AND ALSO OF ALTERNATIVEEMBODIMENTS

A preferred embodiment of the present invention will now be describedwith reference to FIGS. 3( a) and 3(b) and FIGS. 4( a)–4(e). FIG. 3( a)is a cross section view through structure 11 as marked by section line3(a) noted on FIG. 3( b). FIGS. 4( a)–4(e) are also section views takenalong the same section line done for FIG. 3( a) but at later points inthe fabrication of the high impedance surface of the present invention.

In this embodiment a form or structure 11 is fabricated by molding andthe form 11 is subsequently plated with metal and the metal is partiallyremoved to define the capacitor structures. The form or structure 11 ispreferably made by injection molding, in which a mold is filled with aliquid dielectric material, which then hardens into a solid cast whichis removed from the mold. This dielectric material is preferably eithera thermoplastic, which is melted and then injected into the mold andallowed to harden, or a thermoset resin, which is mixed in liquid formfrom two reagents, injected into the mold, and then allowed to harden.The procedure for molding resins is known to those skilled in the art ofinjection molding and therefor is not discussed in further detail here.Important features of the molded structure 11 of this embodiment of theinvention include pre-formed holes or vias 12, which can all be producedin the single molding step, and vertical raised projections or ridges 14that will form a structure for supporting the plates of the capacitors.These projections or ridges 14 may be optionally recessed into thestructure 11 by using a trench 16 as shown in FIG. 3( a). The sidewalls15 of the ridges 14 may be parallel to each other in this embodiment sothe capacitors which will be formed thereon will then have parallelplates. As will be seen, the sidewalls 15 can alternatively betrapezoidal in cross section in order to from non-parallel platecapacitors.

The trenches 16 in this embodiment are optional and are used to make thestructure 11 as thin as reasonably possible. The trenches 16 allow someor all of the capacitors to be recessed somewhat into the structure 11.If not for the trenches 16, the entire length of each capacitor wouldextend above the top major surface of the depicted structure and theheight of the structure 11 would be taller. As such, the trenches 16help make the structure 11 thinner.

The resulting structure 11 includes a grid of projections or ridges 14,which may be square shaped, when viewed in plan view (see FIG. 3( b)),or the grid may be hexagonal, triangular, or have any other desiredshape or pattern when viewed in plan view. Moreover, the projections orridges 14 may have parallel sidewalls 15 as depicted in FIGS. 3( a) and3(b) or may have somewhat non-parallel sidewalls to ease removal of thestructure 11 from its mold. The ridges 14 form cells 20 and each cellsurrounds a region containing: (i) a hole or via 12 that extends to theback side 22 of the structure 11 and (ii), in the depicted embodiment,four adjacent sidewalls 15. In FIGS. 3( a) and 3(b) only nine completecells 20 are shown, but it is to be understood that a complete structure11 would normally comprise hundreds or thousands or even more of suchcells 20. Each cell 20 will help define one of the plates 18 (see FIG.4( b) of four capacitors (an electrically equivalent capacitor C isdepicted in phantom in FIG. 3( b)) associated with each verticalconnection 13 to be formed in the hole or via 12 of the resulting Hi-Zsurface.

Turning now to FIG. 4( a), after the structure 11 of FIGS. 3( a) and3(b) has been formed, preferably by molding, it is plated with thinlayer of metal 24, preferably copper. The copper may be coated withanother metal such as nickel, tin, or gold to provide corrosionresistance, if desired.

Preferably, the entire exterior surface of structure 11 is plated,including the back side 22, the holes 12, and the features 14, 16 on thefront side thereof with metal 24. The thickness of metal 24 is notcritical and might typically be 50 μm or so. The metal that is platedinside the holes 12 creates vertical connections 13 between the metal onthe back side 22 (which will form a ground plane) and the capacitorplates to be defined on the sidewalls 15 of each cell 20 (see FIG. 3(b)). The vertical connections 13 are used to suppress surface waves iscertain embodiments. Those skilled in the art will appreciate the factthat the vertical connections 13 can sometimes be omitted and in suchembodiment the holes 12 can be omitted. The metal that is plated on thesides 15 of the vertical ridges 14 forms vertical capacitor plates 18.The dielectric of the ridges 14 forms the insulator for the capacitors.If the holes 12 have a sufficiently small diameter, the verticalconnections 13 may completely fill holes 12.

The next fabrication step is to pass the structure through a planingdevice, which removes or planes off the tops of the projections orridges 14 as can be seen in FIG. 4( b). This action removes the metalconnections or bridges 26 (see FIG. 4( a)) at the tops of the ridges 14and between adjacent cells 20, so that the plates 18 of the individualcells 20 are now electrically coupled by the metal plating 13 in holes12 only to the lower metal surface 28. This step is important for thecreation of the capacitors and it also provides tunability to thestructure, since the capacitors can be planed to a desired depth, whichdetermines the resonance frequency of the resulting structure. Also,assuming that the structure is not planed to too great a depthinitially, the technique of removing the tops of the ridges 14 allowsfor fine-tuning the capacitance of the resulting structure after otherfabrication steps have been performed to correct for variations inmanufacturing tolerances. Furthermore, different areas of the surfacecan be optionally planed to different depths, to create a surface withareas having different resonance frequencies. This allows a singlesurface to be used for multiple bands of operation. As an alternative,the structure can also be planed or originally molded with a built-intaper 32, as is shown in FIG. 4( c), so that the resonance frequencyvaries smoothly across the surface where such a taper is provided. Inthis embodiment each capacitor ridge 14 has a slightly different averageheight compared to its neighbors. This makes the resonant ground planesurface useful for broadband operation by feeding various areas of thesurface with different antennas according to the desired frequency ofinterest. The ability to tune the surface as a function of position mayalso have applications in producing low-angle radiation from alow-profile antenna, as energy can be coupled into surface waves, whichare then allowed to radiate off the surface after a pre-determineddistance. With any of these surfaces, the final structure may be coatedwith a dielectric layer 36 for purposes of planarization, so that thepossibly delicate fins 34 that form the capacitors are not damaged inuse. See FIG. 4( d) which shows the embodiment of FIG. 4( b) with theadded dielectric layer 36. The fins 34 may be delicate since their sizesare dictated by the frequency at which the resonant surface is to beresonant and when the resonant frequency gets into the gigahertz rangethe feature sizes of the capacitors is rather small (easily viewable bythe human eye, but sufficiently small that the fins 36 may be delicateand therefore it may be desirable to protect them from physical damage.

The embodiments depicted by FIGS. 4( b) and 4(c) both have parallelplate 18 capacitors. As can be seen from FIG. 4( e), if the ridges 14are trapezoidal in cross section when formed, then the capacitor plates18 will be non-parallel. The trenches 16 may also be formed withnon-parallel walls. This embodiment has the advantage that the structure11 more easily releases from its mold (not shown) when molded. As such,non-parallel plate capacitors are preferred for ease of manufacturing.The amount by which the plates are non-parallel may be rather slight andpreferably would only be by an amount needed for ease of manufacturingsince non-parallel plate capacitors tend to make determining the shapeof the taper 32 more complicated (if a taper 32 is utilized). The use ofa taper tends to reduce the capacitance towards the wide end of thetaper thereby requiring taller capacitors in compensation therefor.

Another technique for producing a Hi-Z structure will now be describedwith reference to FIGS. 5( a)–5(f). This embodiment involves buildingvertical capacitors into the structure in connection withdownward-pointing trenches 16. The trenches 16 have sidewalls 15 wherethe plates 18 of the capacitors will be formed and also have trenchbottoms 23 which will be free of metal when the Hi-Z surface of thisembodiment is completely built. In this embodiment, a structure 11 ismolded which bears some resemblance to the structure 11 of FIGS. 3( a)and 3(b). FIG. 5( a) is a cross section view taken through structure 11along section line 5(a) in the plan view depicted by FIG. 5( f). In thisembodiment there is no need for ridges 14—rather a grid of trenches 16is formed when molding the structure 11.

Turning to FIG. 5( b), a wire grid 17 is laid into the trenches 16 toprevent the capacitors from being shorted out when the structure 11 iscoated with a layer 24 of metal. Layer 24 is preferably formed by firstevaporating a thin metal layer 24-1 onto structure 11, covering everypart of the surface except that which is covered by the wire grid 17 asshown by FIG. 5( c). For this initial metal layer evaporation, it ispreferable to evaporate a metal that has low thermal conductivity andthat can provide a base for electroplating more metal. Nickel is acommon choice for an evaporated metal. After the evaporation step, thewire grid 17 is removed. The aforementioned metal evaporation steppreferably lays down a very thin layer of metal 24-1 which will not forma connection across junctions where there is no line-of-sight from theevaporation source. Hence, the wire grid 17 will not become attached tometal 18-1 on the sidewalls 15 of the trenches 16 as long as thediameter of the wires of grid 17 is somewhat smaller than the width ofthe trenches 16.

After evaporation and wire grid removal, other metals are preferablyelectroplated onto the exposed metal as shown by FIG. 5( d) forming athicker metal layer 24 (and thicker metal 18 on the sidewalls of thetrenches 16). The first metal which is preferably electroplated to theexposed nickel (for example) layer 24-1 is a metal layer having a highelectrical conductivity (such as copper). The exposed high conductivitymetal layer (preferably copper) is then preferably covered with anotherlayer that will provide corrosion resistance. Nickel, tin, or gold arecommon choices for metals for the corrosion resistant layer. The initiallayer 24-1 and the added layers of a low thermal conductivity metal(preferably nickel), a high electrical conductivity metal (preferablycopper) and a corrosion resistance layer (preferably nickel, tin orgold) are collectively identified as layer 24 in FIGS. 5( d) and 5(e).

The resulting structure of FIG. 5( d) contains the capacitor plates 18formed from the metal coated on the sidewalls of the trenches 16 as wellas the vertical connections 13 formed in the vias 12. Turning now toFIG. 5( e), trenches 19 may also built into the bottom of structure 11,forming an “inverse waffle” structure, which would have improvedmechanical flexibility. In this case, the capacitors should be filledwith a dielectric 21 so that their capacitance will not change when theHi-Z surface is bent or flexed. This embodiment has several drawbackscompared to the preferred embodiments of FIGS. 3( a), 3(b) and4(a)–4(e), including lack of tunability, and sensitivity to flexingunless the air capacitors are filled with dielectric 21.

In FIGS. 5( f) only nine complete cells 20 are shown, but it is to beunderstood that the structure 11 would normally comprise hundreds orthousands or more of such cells 20. Each cell 20 will help define one ofthe plates 18 of four capacitors (an electrically equivalent capacitor Cis depicted in phantom in four places in FIG. 5( f)) associated witheach vertical connection 13 of the resulting Hi-Z surface.

For the preferred embodiments of FIGS. 3( a), 3(b) and 4(a)–4(e), thecapacitors are filled with a dielectric. If the dielectric is relativelyinelastic, then if the surface is bent or flexed, it is the width of thetrenches that will change, and not the width of the capacitors. In thecase of the wire grid constructed embodiment of FIGS. 5( a)–5(d), thecapacitors may be only filled with air, while the rest of the structureis filled with dielectric. If this structure is bent or flexed, then theregions filled with the most compressible material (air) will be theregions that are deformed. This would result in a large change incapacitance in response to bending the surface. For most applications,such a change in capacitance is undesirable and, for this reason, it isusually important to fill the capacitors with another material 21 thatis preferably less elastic than the plastic which provides the rest ofthe structure if (1) the structure is subject to bending or flexing and(2) having the structure change capacitance in response thereto would beundesirable.

Yet another embodiment of a Hi-Z structure is now described withreference to FIGS. 6( a)–6(e). This embodiment takes advantage of thefact that the capacitive metal plates of the Hi-Z surface are easy tofabricate using photolithography and standard printed circuit boardtechniques, while the vias are easy to produce en masse using injectionmolding techniques. This Hi-Z structure is made by a hybrid of twotechnologies: injection molding and printed circuit technology. Turningto FIG. 6( a), the lower part of the Hi-Z surface is a structure 11 withvias 12 is fabricated using injection molding technology. Alternativelythe vias 12 could be stamped into a substrate or formed therein using anarray of pins or drills. As is shown by FIG. 6( b), the structure 11 isthen coated with metal, preferably copper, to make the back side 22 andthe vias 12 conductive, and then patterned to remove metal from thefront side except from pads 23 adjacent vias 12 which pads 23 will beused for solder bonding. Solder is then flowed onto the front of thestructure to form solder bumps 34 on the tops of the pads 23 adjacentvias 12 (see FIG. 6( c)). A second layer of dielectric 36, which ispreferably provided by a printed circuit board 36, is patterned usingstandard photolithographic processing to pattern the metal disposedthereon as is shown by FIG. 6( d) to form an array of plates 10 a on anupper surface thereof and an array of plates 10 b on the lower surfacethereof. The structure of FIG. 6( c) and the patterned printed circuitboard 36 are aligned and the two structures 11, 36 are heated to bondthem together as is shown in FIG. 6( e). The resulting Hi-Z surface hasthe advantage that the capacitors can be easily defined byphotolithographic processing of the metal on printed circuit dielectric36, while the vias 12 can be easily formed when structure 11 is formedby injection molding. This hybrid structure takes advantage of thestrengths of each fabrication method. The final structure can later becoated with tin, nickel, or gold for corrosion resistance, if desired.

Instead of forming structure 11 by injection molding, structure 11 ofany of the previously described embodiments can be formed from apre-fabricated sheet of dielectric which is processed with a hot press,in which an array of hot metal pins are forced through the structure toform the holes and other surfaces are used to form any trenches orprojecting walls needed. Like injection molding, this technique has theadvantage that many holes can be formed quickly. The hot press methodhas the additional advantage that it uses a pre-formed dielectric sheet,in which the thickness can be specified very accurately.

Still another embodiment of a Hi-Z surface is now described withreference to FIGS. 7( a)–7(d). In this embodiment metal stamping is usedto make a low-cost Hi-Z surface by forming the capacitors and vias in asingle stamping process of a metal sheet 38. FIG. 7( a) depicts a moldhaving reciprocating mold surfaces 40 and 41 for forming the shape ofthe desired structure, which mold surfaces include regions 42 to shearoff certain areas of the sheet metal to help define the plates 18 of thecapacitors. The mold also includes elongated regions 44 that formvertical protrusions 45 in the sheet metal.

The mold is used to stamp the sheet 38 as shown by FIG. 7( b). Thisstamped sheet metal 38 is then removed from one section of the mold andapplied (see FIGS. 7( c) and 7(d)) to a second flat sheet of metal 39,to which it is then connected by soldering or spot welding at points 46where the two sheets meet. The completed structure can then be filledwith a dielectric for mechanical support, if desired. One cell 20 isdepicted by phantom line 20 and a typical capacitor is depicted byreference number 18. In plan view the completed structure would comprisehundreds or thousand or more of such cells. The cells 20 in thisembodiment would have a square shape in plan view, but other shapescould be used just as well as a matter of design choice.

In all of the embodiments disclosed herein, only a few capacitors aredepicted since the figures depict the structures considerably enlargedfor ease of understanding and illustration. It is to be understood thata typical Hi-Z surface will have hundreds, thousands or even morecapacitors. The terms ridges-and projections are used hereinsynonymously to refer to element 14.

Common reference numbers are sometimes used herein to refer to objectswhich have similar features and/or functions, but which may not beidentical to each other.

1. A method of making a high impedance surface comprising the steps of:(a) forming a structure from sheet metal, the structure having aplurality of openings therein with confronting pairs of sidewalls on thesides of the openings, the structure also having a plurality ofprotrusions projecting from a major surface thereof; and (b) joiningsaid structure to additional sheet metal such that ends of saidprotrusions remote from said major surface are coupled to the additionalsheet metal, the confronting pairs of sidewalls of each of said openingsdefining opposing plates of a capacitor for controlling resonancefrequencies of said high impedance surface as a function of location ofeach capacitor along said high impedance surface.
 2. The method of claim1 wherein the additional sheet metal is a generally planar sheet metal.3. The method of claim 1 wherein the protrusions have a greater depththan do the sidewalls.
 4. The method of claim 1 wherein the sidewallsare spaced a distance from the additional sheet metal.
 5. The method ofclaim 1 wherein the sidewalls which confront one another are disposedparallel to each other.
 6. The method of claim 1 wherein said sidewallsdefine a repeating geometric pattern.
 7. The method of claim 6 whereinthe repeating geometric pattern is a pattern of square-shaped cells. 8.A method of making a high impedance surface comprising the steps of: (a)forming a structure from sheet metal, the structure having a pluralityof openings therein with confronting sidewalls on the sides of theopenings, the structure also having a plurality of protrusionsprojecting from a major surface thereof; and (b) joining said structureto additional sheet metal such that ends of said protrusions remote fromsaid malor surface are coupled to the additional sheet metal wherein theconfronting sidewalls are formed to depend in a direction away from saidopenings and towards said additional sheet metal, but being spacedtherefrom, so that a gap occurs between each depending sidewall and saidadditional sheet metal.
 9. A method of making a high impedance surfacehaving a plurality of capacitors formed therein, the method comprising:(a) forming a structure from sheet metal, the structure having aplurality of openings therein with confronting sidewalls defining thesides of the openings, the structure also having a plurality ofprotrusions projecting from a major surface thereof, the confrontingsidewalls providing opposing plates of said capacitors; and (b) joiningsaid structure to additional sheet metal such that ends of saidprotrusions remote from said major surface are coupled to the additionalsheet metal.
 10. The method of claim 9 wherein the additional sheetmetal is a generally planar sheet metal.
 11. The method of claim 9wherein the protrusions have a greater depth than do the sidewalls. 12.The method of claim 9 wherein the sidewalls are spaced a distance fromthe additional sheet metal.
 13. The method of claim 9 wherein thesidewalls which confront one another are disposed parallel to eachother.
 14. The method of claim 9 wherein said sidewalls define arepeating geometric pattern.
 15. The method of claim 14 wherein therepeating geometric pattern is a pattern of square-shaped cells.
 16. Themethod of claim 9 wherein the confronting sidewalls depend in adirection away from said openings and towards said additional sheetmetal, but being spaced therefrom, so that a gap occurs between eachdepending sidewall and said additional sheet metal.